This primarily to suppress error such as:
/var/tmp/portage/media-libs/libsfml-2.6.0/work/SFML-2.6.0/src/SFML/Window/Unix/JoystickImpl.cpp:405:36: error: implicit conversion changes signedness: 'unsigned long' to 'int' [-Werror,-Wsign-conversion]
            int result = ioctl(fd, JSIOCGNAME(sizeof(name)), name);
                         ~~~~~     ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/linux/joystick.h:59:26: note: expanded from macro 'JSIOCGNAME'
#define JSIOCGNAME(len)         _IOC(_IOC_READ, 'j', 0x13, len)                 /* get identifier string */
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Even seems like upstream is has removed it with commit
https://github.com/SFML/SFML/commit/aa82ea132b9296a31922772027ad5d14c1fa381b
--- a/cmake/CompilerWarnings.cmake
+++ b/cmake/CompilerWarnings.cmake
@@ -68,16 +68,11 @@ function(set_file_warnings)
         # -Wimplicit-fallthrough # warn when a missing break causes control flow to continue at the next case in a switch statement (disabled until better compiler support for explicit fallthrough is available)
         ${NON_ANDROID_CLANG_AND_GCC_WARNINGS}
     )
-    
+
     # Disable warnings as errors when using Clang on Windows to work around deprecation warnings in Windows APIs
     if(SFML_OS_WINDOWS AND (SFML_COMPILER_CLANG OR SFML_COMPILER_CLANG_CL))
         set(WARNINGS_AS_ERRORS FALSE)
     endif()
-    
-    if(WARNINGS_AS_ERRORS)
-        set(CLANG_AND_GCC_WARNINGS ${CLANG_AND_GCC_WARNINGS} -Werror)
-        set(MSVC_WARNINGS ${MSVC_WARNINGS} /WX)
-    endif()
 
     set(CLANG_WARNINGS
         ${CLANG_AND_GCC_WARNINGS}